To manufacture large-area semiconductor components, in particular solar cells or thin-film transistor arrays, a cost reduction is aimed at by establishing an economical manufacturing process. This is attempted by depositing the semiconductor substance in the form of a thin film on inexpensive substrates. In the case of amorphous silicon as the semiconductor film, considerable efforts have been made, but despite successes, the problem of stability of the components is still only unsatisfactorily solved.
As an alternative, there is the possibility of depositing polycrystalline silicon on a substrate in the form of thin films. It is expected that this will largely eliminate any stability problems.
In conventional crystalline silicon solar cells the silicon wafer thickness is greater than 200 .mu.m in order to avoid minority carrier recombination at the rear contact and effective absorption of the sunlight.
Gotzberger, Proc. 15th IEEE Photovolt. Spec. Conf., 1981, Kissimee, Fla., USA, p. 867, shows a way of achieving an efficient light absorption, even with film thicknesses of 5-20 .mu.m, using a diffuse rear reflector. Knobloch, Voss and Gotzberger, Proc. 16th EC Photovolt. Solar Energy Conf. Apr. 15-19, 1985, London, p. 285, calculate a theoretically possible efficiency of 25% for a crystalline thin-film solar cell.
In order for the light-generated charge carriers reach the electrical field of the semiconductor element (in particular the solar cell), the diffusion length of the base material must be sufficiently high.
In the calculations of Knobloch, Voss and Gotzberger, it is shown that diffusion lengths L.sub.D' =50 .mu.m for film thicknesses of 20 .mu.m, L.sub.D =30 .mu.m for film thicknesses of 10 .mu.m, and L.sub.D =20 .mu.m for film thicknesses of 5 .mu.m are necessary to achieve a good collection efficiency. Since recombination centers form at the surfaces and grain boundaries in polycrystalline silicon, two conditions must be met for good functioning of a thin-film solar cell to be achieved: firstly, a good passivation of the boundary surfaces (surface passivation, in particular rear face passivation), and secondly, grain boundary passivation or large grains with G=2-4.multidot.L.sub.D (G=grain size). From the above are obtained the requirements for a grain size of 100 .mu.m-200 .mu.m for polycrystalline silicon in the thin-film solar cell and for an effective and stable passivation of the grain boundaries. In addition, the grain boundaries must be aligned perpendicularly to the substrate surface.
In addition, the front and rear faces of the cell must be passivated by suitable measures.
The practical achievement of this concept encounters difficulties of a fundamental nature, since the high nucleation rate results in columnar and fine-grained growth during the deposition of thin crystalline silicon films on non-monocrystalline substrates.
The methods frequently described in the literature for deposition of silicon films, such as electron beam evaporation, P. H. Fang, C. C. Schubert, J. H. Kinnier and Dawen Pang, Appl. Phys. Lett. 39 (1981) 256; Charles Feldmann, N. A. Blum and F. Satkiewicz, Proc. 14th IEEE Photovolt. Spec. Conf. 1980, San Diego, p. 391; R. J. C. van Zolingen and A. H. Kipperman, Thin Solid Films, 58 (1979), and thermal CVD methods by reduction of various chlorosilane compounds with hydrogen at temperatures of 1100.degree.-1300.degree. C., lead to growth of grains of max. 20-30 .mu.m size.
By long tempering at higher temperatures, a certain degree of recrystallization in silicon films can lead to the formation of larger grains, C. Daey Ouwens and H. Heijligers, Appl. Phys. Lett. 26 (1975) 579. In this case, grain sizes of up to 100 .mu.m are achieved, but only with tempering times of 10 hours and at temperatures of more than 1350.degree. C. Under these conditions, heavy diffusion of impurities from the substrate into the silicon film must be expected, which is detrimental to the quality of the solar cell.
Considerably larger crystals are obtained by epitaxially depositing silicon films onto multicrystalline silicon wafers with millimeter grain size, T. Warabisako, T. Saitoh, H. Itoh, N. Nakamura and T. Tokuyama, Jpn. J. Appl. Phys., 17 Suppl. (1978) 309; P. H. Robinson, R. V. D'Aiello, D. Richman and B. W. Faughnan, Proc. 13th IEEE Photovolt. Spec. Conf., 1978, Washington D.C., p. 1111.
T. Warabisako et al., supra, use substrates made from polycrystalline silicon of metallurgical purity, of Czochralski type and sawn into 0.4 .mu.m thick wafers. On these wafers, a 20-30 .mu.m thick p-Si film is deposited by reduction of SiH.sub.2 Cl.sub.2 with H.sub.2 at 1100.degree.-1150.degree. C., followed by deposition of a 0.5 .mu.m thick n.sup.+ -doped Si film at 1000.degree.-1050.degree. C. A similar process is applied by P. H. Robinson et al., supra. In this process, however, the economic advantage is lost, since the silicon wafers must be manufactured separately.
T. L. Chu, S. S. Chu, K. Y. Duhand H. C. Mollenkopf, J. Appl. Phys. 48 (1977) 3576, employ as the basis for epitaxial silicon deposition a silicon film of approx. 200-500 .mu.m thickness (p-doped, 0.002-0.004 .OMEGA. cm) on a graphite substrate. This film is generated by reduction of SiHCl.sub.3 with H.sub.2
To manufacture large grains, the films are melted and crystallized in a second process. An obstacle here is the high surface tension of silicon, which results in the melted material on the substrate contracting into droplets, thus interrupting the melt zone. However, by melting of a narrow zone, it is possible to get around this effect. The reactor comprises a quartz tube which is flushed with hydrogen and heated with a radio-frequency coil. The Si-coated substrate is passed underneath an inductively heated SiC-coated graphite rod (heater) located inside the reactor. The hot graphite rod radiates heat and ensures a melted zone that is 2 cm wider in the middle than at the edge of the substrate. After crystallization of Si films of this type, a solar cell is manufactured by epitaxially depositing a 20-30 .mu.m thick p-conducting silicon film on the crystallized base, and then depositing a 0.2-0.4 .mu.m thick n-conducting emitter film on top of it.
According to M. Kerber, M. Bettini and E. Gornick, Proc. 17th IEEE Photovolt. Spec. Conf., 1984, Kissimee, Fla., p. 275, a highly doped n-conducting Si film is first deposited on a graphite substrate and is then melted and recrystallized using a tungsten wire passed over it as a radiation heater. The optimum thickness of this film is given as 20-30 .mu.m. The photovoltaic active n-conducting film with 60-100 .mu.m thickness is now epitaxially deposited. An SIS solar cell with 9% efficiency is then manufactured by a subsequent spray pyrolysis of SnO.sub.2.
Common to both methods is the fact that the heater for the melting zone in the reaction chamber is under inert gas (H.sub.2). It is therefore necessary to interrupt the coating process during melting, otherwise the heaters in the reactor would become Si-coated.
The recrystallized silicon films described in both methods are too thick for manufacture of a thin-film solar cell as mentioned above from polycrystalline silicon of only 20-50 .mu.m thickness. Also, the films do not have the necessary lattice perfection for a photovoltaically usable material, with the result that a very thick epitaxial film with the required photoelectrical properties must be deposited.
As a result, the economic advantage possible in principle with deposition from the gas phase is lost because:
1. a thick film has to be used to make large crystals, which however have too poor a lattice quality for photovoltaic purposes; and PA1 2. a thick film has to be deposited onto it because there is no rear reflector to absorb the sunlight effectively.
Furthermore, our own tests with isostatically compressed, high-density graphite substrates (12% porosity) have shown that the graphite soaks up the silicon when it is melted thereon, so that thin silicon films disappear into the graphite and hence cannot be recrystallized. A thin-film solar cell cannot therefore be manufactured in this way.
Finally, DE-B 1 223 951 describes a method of manufacture of a semiconductor component with at least one pn-junction, in which a nucleation film is deposited onto a substrate by melting. The material of the carrier and/or impurities can diffuse into the semiconductor material so that the semiconductor component is not pure and stable when viewed in the long term.